It is characteristic of a modern mobile communication system that a mobile station is free to roam and connect from one cell to another within the mobile communication system. If a mobile station does not have an ongoing call, the cell crossover only results in registering to a new cell. If a mobile station MS is handling a call during the cell crossover, the call must also be switched to the new cell by a way which causes as little disturbance to the call as possible. The cell crossover process during an ongoing call is called a handover. A handover may also be carried out within a cell from one traffic channel to another. In order for the mobile communication system to be able to detect a need for a handover and to select a suitable target cell for the handover, various kinds of measurements are required in order to determine the connection quality and field strength levels of the neighboring cells. A handover from the serving cell to a neighboring cell can occur, for example, (1), as measurement results of the mobile station/base station indicate a low signal level and/or quality in the present serving cell, and a better signal level can be obtained in a neighboring cell, or (2), as a neighboring cell enables communication at lower transmitting power levels. The latter may occur in cases in which a mobile station is in a border area between cells.
In order to ensure stability of the mobile communication network, the measurement results and the parameters used in the handover are averaged over a period of time. As a result, the handover is made less sensitive to measurement results distorted by instantaneous interference or fadings.
A problem occurring in this kind of a mobile communication network relates to mobile stations which move at high speed in relation to cell size. When a mobile station moves at high speed in the cell and goes behind an obstacle that shadows the signal from the serving cell, the field strength of the serving cell received by the mobile station suddenly drops, but the mobile communication network is not able to react fast enough so that the call might be maintained. Thus, the call may break. Some mobile communication systems employ a so-called call re-establishment procedure which keeps the call connected in the fixed network for a certain period of time even though the connection to the mobile station breaks. If the mobile station can re-establish a connection to the fixed radio network during this time through a cell, the call can quickly be continued. This is not, however, a good solution because there must first be a break in the radio connection and, consequently, the call, before a new connection can be established. This problem which relates to fast-moving mobile stations and slow handovers is especially well-marked in microcell networks.